1. Field of the Invention
The present invention relates to osmotic delivery systems for delivering beneficial agents, and more particularly, to an osmotic delivery system having an osmotic engine and a membrane plug allowing fluid to pass into the osmotic engine.
2. Description of the Related Art
Controlled delivery of beneficial agents, such as drugs, in the medical and veterinary fields has been accomplished by a variety of methods. One method for controlled prolonged delivery of beneficial agents involves the use of osmotic delivery systems. These systems can be implanted within a body of a human or animal to release beneficial agents in a controlled manner over a preselected time or administration period. In general, osmotic delivery systems operate by imbibing liquid from the outside environment and releasing corresponding amounts of the beneficial agent.
A known osmotic delivery system, commonly referred to as an "osmotic pump," generally includes some type of a capsule or enclosure having a semipermeable portion which selectively passes water into an interior of the capsule containing a water-attracting osmotic agent. In one known osmotic delivery system the walls of the capsule are substantially impermeable to items within and outside the capsule. A membrane plug is inserted into one end of the capsule and acts as the semipermeable portion to allow water to pass into the interior of the capsule. The difference in osmolarity between the water-attracting osmotic agent and the environment surrounding the capsule causes water to pass through the membrane plug into the capsule which in turn causes the beneficial agent within the capsule to be delivered through a delivery orifice. The water-attracting osmotic agent may be the beneficial agent delivered to the patient; however, in most cases a separate osmotic agent is used specifically for its ability to draw water into the capsule.
When a separate osmotic agent is used, the osmotic agent may be separated from the beneficial agent within the capsule by a movable dividing member or piston. The structure of the capsule is such that the capsule does not expand when the osmotic agent takes in water and expands. As the osmotic agent expands, it causes the piston to move and the beneficial agent to be discharged through the delivery orifice at the same rate as the liquid, which is typically water, enters the osmotic agent by osmosis. Osmotic delivery systems may be designed to deliver a beneficial agent at a controlled constant rate, a varying rate, or in a pulsatile manner.
In the known osmotic delivery systems an osmotic tablet is generally used as the osmotic agent and is placed inside the capsule adjacent the piston. The membrane plug is placed in an opening in the capsule through which the tablet and piston were inserted. Known membrane plugs are typically cylindrical members which seal the interior of the capsule from the exterior environment, permitting only certain liquid molecules from the environment of use to permeate through the membrane plug into the interior of the capsule. The rate that the liquid permeates through the membrane plug controls the rate at which the osmotic agent expands and drives the beneficial agent from the delivery system through the delivery orifice. The rate of delivery of the beneficial agent from the osmotic delivery system may be controlled by varying the size of the beneficial agent delivery orifice, the osmotic material, a size and shape of the membrane plug, or the permeability coefficient of the membrane plug.
The permeability coefficient of a membrane plug is dependent on the particular material or combination of materials used in the plug. Thus, the delivery rate of the beneficial agent may be controlled by forming the same configuration membrane plug from different semipermeable materials having permeability coefficients which result in delivery of the beneficial agent at a desired delivery rate. One problem associated with obtaining different permeation rates in this manner is that a different membrane material must be used for every system which has a different desired beneficial agent delivery rate, requiring the purchase of many different membrane materials and manufacture of many different membrane plugs.
Many osmotic delivery systems which use membrane plugs have problems with expulsion of the membrane plug from the capsule. Expulsion may occur after the beneficial agent has been completely delivered when the osmotic agent continues to draw water into the capsule and forces the membrane plug out of the capsule. Some osmotic delivery systems use glues or adhesives to prevent the capsule from leaking and to ensure that the membrane plug remains in place to prevent harmful materials from the interior of the capsule from leaking into the surrounding environment. In addition to adding a manufacturing step and increasing costs, applying an adhesive to the membrane plugs may affect the rate of permeation.
Membrane plugs used in systems which are designed to deliver a beneficial agent at delivery rates which allow complete delivery of the beneficial agent in time periods from about 1 day to 2 weeks are particularly susceptible to membrane expulsion problems. These rapid delivery membranes swell due to water uptake within hours of implantation and become slippery and sponge like. The rapid swelling of such membranes tends to cause the membranes to be expelled from the capsule.
Because of the above-identified problems associated with current osmotic delivery system membrane plugs, it is difficult and expensive to provide osmotic delivery systems which administer beneficial agents at different desired delivery rates and prevent expulsion of the membrane plug.